North Korea fired three ballistic missiles into the sea off its east coast on Monday, the South Korean and US militaries said, as the leaders of the Group of 20 major economies held a summit in China, the North's main diplomatically.

The missiles likely landed in the sea 200 to 250 km (120 to 160 miles) west of Hokkaido, Japan's northern-most main island.

"The members of the Security Council deplore all Democratic People’s Republic of Korea ballistic missile activities, including these launches," the UN body said in a statement, using North Korea's official name.

Earlier, the United States called for action to enforce Security Council resolutions on North Korea prohibiting ballistic missile-related activities, Reuters reported.

"The Security Council must remain unequivocal and united in the condemnation of these tests and we must take action to enforce the words we put on paper, to enforce our resolutions," said US ambassador to the United Nations, Samantha Power, after a Security Council meeting.

Power, who spoke along with her Japanese and South Korean counterparts, refrained from elaborating on what further action the Security Council could take.

Hahn Choong Hee, South Korea's deputy permanent representative to the United Nations, said Pyongyang was spending "a considerable amount" of its resources in developing weapons of mass destruction while sacrificing the living conditions of North Koreans.

Monday's missile launches were the latest in a series by North Korea this year in violation of Security Council resolutions that were supported by China banning ballistic missile-related activities by Pyongyang.

North Korea rejects the ban as infringing its sovereign right to pursue a space program and self-defense.

Asked whether China agreed more significant measures needed to be taken, permanent British representative to the United Nations, Matthew Rycroft, said, "We're talking to all of our council colleagues."

North Korea has been under UN sanctions since 2006. The 15-member Security Council toughened the sanctions in March in response to North Korea's fourth nuclear test in January and the launch of a long-range rocket in February.

The statement from the Security Council said members would closely monitor the situation and "take further significant measures in line with the Council's previously expressed determination" without elaborating further.